(1) Field of the Invention
This invention relates to a grinding process monitoring system incorporated into a grinding apparatus for grinding substrates such as semiconductor wafers, the monitoring system being operable to monitor progress of a grinding process by optically measuring the thickness of a film applied to a surface of a substrate and undergoing the grinding process. The invention relates also to a method of monitoring the grinding process.
(2) Description of the Related Art
With an ever-enlarging scale of integration of semiconductor devices taking place today, circuit wiring has become increasingly minute to diminish spaces between conducting paths. In the case of a photolithographic process for fabricating such circuit boards, the surfaces of semiconductor wafers (substrates) must be smoothed because of reduced focal depths. As one smoothing method, a grinding apparatus is used to smooth a film (insulating film) applied to the surface of a substrate and grind the film down to a target thickness.
In a grinding process performed with such a grinding apparatus, a top ring supports the substrate and presses its surface against an abrasive cloth extended over a turntable. The turntable and top ring (substrate) are rotated independently of each other while a slurry (abrasive solution) is supplied to the abrasive cloth.
This type of grinding apparatus includes a monitoring system for monitoring progress of the grinding process. A monitoring system has been proposed recently for optically measuring the thickness of a film being ground. This monitoring system measures the current thickness of the film applied to the substrate surface and undergoing the grinding process, by irradiating the substrate surface with measuring light, taking in the measuring light reflected from the substrate surface, measuring the spectrum of the reflected light, and analyzing the spectrum measured of the reflected light.
However, abrasive powder may remain adhering to the substrate surface (or the film being ground) while the film thickness is measured. Such abrasive powder absorbs or scatters the measuring light to make accurate thickness measurement impossible.
Inventors have carried out film thickness measuring experiments aimed at solving the above problem. The measuring system used in the experiments had a measuring window opposed to the surface of a substrate in time of measuring film thickness. Through this window, measuring light was emitted to the surface of the substrate and the light reflected from the substrate surface was taken in. A measurement region between the substrate surface and the measuring window was filled with cleaning liquid (deionized water) to form a liquid curtain therein.
According to the system having the above construction, abrasive powder adhering to the substrate surface was washed away by the liquid curtain formed in the measurement region, to the extent no longer obstructive to the film thickness measurement. This prevented absorption and scattering of the measuring light, offering a prospect for an accurate measurement of the thickness of a film being ground.
However, it has been found that, when the above technique of measuring film thickness by forming a liquid curtain is applied to an actual grinding apparatus, the cleaning liquid could produce bubbles in the measurement region. Such bubbles are detrimental to an accurate measurement of film thickness.